<p>Severity of subacute thyroiditis (SAT), an inflammation of the thyroid gland caused by a viral infection, is variable. The objective of this observational, retrospective study was to correlate severity of SAT with viral epidemics. 402 SAT patients were evaluated at the Endocrinology Unit of the University Hospital of Pisa, Italy, from January 2011 to December 2020. Data about distribution of SAT in the four quarters of each year were collected. Severity of thyrotoxicosis and of inflammation and rate of hypothyroidism in each quarter were evaluated. Distribution and severity of SAT were correlated with the influenza seasons and the COVID-19 outbreaks in Italy. Most SAT cases occurred in the 3rd quarter of all years, except in 2012 and 2015, in which cases they were more frequent in the 1st quarter, and in 2020, in which they were more common in the 2nd and the 4th quarters. SAT cases occurring in these diverging quarters were characterised by more severe thyrotoxicosis and inflammation and a higher rate of hypothyroidism. The year 2012 was characterised by an almost exclusive circulation of the A/H3N2 virus, the year 2015 by a poor efficacy of the vaccine against the A/H3N2 virus, the 2nd and 4th quarters of 2020 by the SARS-CoV2 outbreaks. SAT severity depends on the aetiological agent, some strains of influenza viruses and SARS-CoV-2 causing more severe clinical forms compared to those observed in the 3rd quarter, which are typically related to enterovirus infections. These results suggest a relationship between the epidemiology of circulating viruses and the clinical severity of SAT. These data are consistent with the hypothesis that SAT cases related to some strains, such as A/H3N2 and SARS-CoV-2, may be more severe than those associated with enterovirus seasons.</p>